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Magnesium Isotope Constraints on the Holocene Hydromagnesite Formation in Alkaline Lake Dujiali, Central Qinghai-Tibetan Plateau

Published version
Peer-reviewed

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Abstract

jats:titleAbstract</jats:title>jats:pHydromagnesite <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrf21678-math-0001.png" xlink:title="urn:x-wiley:21699003:media:jgrf21678:jgrf21678-math-0001" /> is a common hydrated magnesium carbonate mineral found in alkaline lakes on Earth, potentially present on Mars, and is also a key mineral for carbon capture and storage. However, mechanisms governing its formation in alkaline lakes remain enigmatic. Extensive hydromagnesite formed during the Holocene in the alkaline Dujiali Lake (DL), central Qinghai‐Tibetan Plateau, making it an ideal field site to constrain the process of hydromagnesite formation in a modern environmental context. In this study, we report a set of magnesium isotope ratios (<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrf21678-math-0002.png" xlink:title="urn:x-wiley:21699003:media:jgrf21678:jgrf21678-math-0002" /> expressed as jats:italicδ</jats:italic>jats:sup26</jats:sup> jats:styled-contentMg</jats:styled-content>) data from DL on abiotic hydromagnesite (mean = −1.35‰ ± 0.14‰) modern lake waters (−0.07‰ to +0.46‰), and rivers and groundwater (−0.53‰ to −1.46‰). These differences in jats:italicδ</jats:italic>jats:sup26</jats:sup> jats:styled-contentMg</jats:styled-content> (and also Mg/Ca) are most likely caused by low‐Mg carbonate precipitation, a process which fractionates Mg/Ca and <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrf21678-math-0003.png" xlink:title="urn:x-wiley:21699003:media:jgrf21678:jgrf21678-math-0003" /> values. A semi‐quantitative box model of the lake chemistry was developed based on carbonate equilibria to investigate the behavior of Mg isotopes during the evolution of the lake chemistry. The modeling results indicate that evaporation concentrates solutes in the lake driving saturation of multiple minerals. Aragonite reaches saturation before hydromagnesite, preferentially removing Ca relative to Mg via aragonite precipitation. This process elevates the Mg/Ca of the lake and the saturation index of hydromagnesite, increasing the likelihood of hydromagnesite formation. Given that the Mg/Ca ratio of many alkaline lakes is far below than that required for the formation of hydromagnesite, our findings suggest that low‐Mg carbonate precipitation may be a common precursor process for abiotic hydromagnesite precipitation in evaporative environments in addition to a high Mg source likely derived from ultramafic rocks.</jats:p>

Description

Funder: Observation and Research Station of Salt Lakes in Tibetan Plateau

Keywords

37 Earth Sciences, 3709 Physical Geography and Environmental Geoscience, 3703 Geochemistry, 3705 Geology

Journal Title

Journal of Geophysical Research: Earth Surface

Conference Name

Journal ISSN

2169-9003
2169-9011

Volume Title

Publisher

American Geophysical Union (AGU)
Sponsorship
NERC (NE/T007214/1)
NERC (NE/V012037/1)
Natural Environment Research Council (2262717)
Natural Environment Research Council (NE/M001865/1)
NERC (NE/S007164/1)